Motor learning induces astrocytic hypertrophy in the cerebellar cortex

Abstract

Motor skill learning, but not mere motor activity, is associated with an increase in both synapse number and glial cell volume within the cerebellar cortex. The increase in synapse number has been shown to persist for at least 4 weeks in the absence of continued training. The present experiment similarly examined how a prolonged interruption in training affects the training-induced increase in astrocytic volume. Adult female rats were randomly allocated to either an acrobatic motor learning condition (AC) or a motor control condition (MC). The AC animals were trained to traverse a complex series of obstacles and each AC animal was pair matched with an MC animal that traversed an obstacle-free runway. These groups were further assigned to one of three training conditions. Animals in the early condition were trained for 10 consecutive days, animals in the delay condition received the same 10 days of training followed by a 28-day period without training, and animals in the continuous condition were trained for the entire 38 days. Unbiased stereological techniques were used to determine that AC animals had a significantly greater volume of astrocytes per Purkinje cell in the cerebellar paramedian lobule than the MC animals, a difference which was reduced (and not statistically detectable) among animals in the delay condition. These findings demonstrate that learning triggers the hypertrophy of astrocytic processes and furthermore that, unlike learning-induced synaptogenesis, astrocytic growth is reduced in the absence of continued training.

Figures

Fig. 1

A. Electron micrograph taken within the molecular layer of the cerebellar paramedian lobule. B. The same micrograph is shown with glial profiles shaded in gray. Astrocytic volume fraction was estimated by overlaying a grid of equi-distant points, counting the total number of points (line intersections) located within astrocytic profiles, and then dividing by the total number of points on the micrograph (see text).

Fig. 2

A. Mean time/trial/task (±SEM) on the motor learning apparatus. Despite a four week break from training, animals in the DELAY condition performed as well as animals in the CONTINUOUS condition (reprinted from Kleim et al., 1997). B. Motor learning induces hypertrophy of astrocytes in the cerebellar cortex. The volume of astrocytes per Purkinje cell (± SEM) within the cerebellar paramedian lobule was greater in AC animals compared to MC animals (p<.01>